The rising cost of warming waters: effects of temperature on the cost of swimming in fishes

Impacts of ocean warming on fish size reductions on the world's hottest coral reefs

The impact of ocean warming on fish and fisheries is vigorously debated. Leading theories project limited adaptive capacity of tropical fishes and 14-39% size reductions by 2050 due to mass-scaling limitations of oxygen supply in larger individuals. Using the world's hottest coral reefs in the Persian/Arabian Gulf as a natural laboratory for ocean warming - where species have survived >35.0 °C summer temperatures for over 6000 years and are 14-40% smaller at maximum size compared to cooler locations - we identified two adaptive pathways that enhance survival at elevated temperatures across 10 metabolic and swimming performance metrics. Comparing Lutjanus ehrenbergii and Scolopsis ghanam from reefs both inside and outside the Persian/Arabian Gulf across temperatures of 27.0 °C, 31.5 °C and 35.5 °C, we reveal that these species show a lower-than-expected rise in basal metabolic demands and a right-shifted thermal window, which aids in maintaining oxygen supply and aerobic performance to 35.5 °C. Importantly, our findings challenge traditional oxygen-limitation theories, suggesting a mismatch in energy acquisition and demand as the primary driver of size reductions. Our data support a modified resource-acquisition theory to explain how ocean warming leads to species-specific size reductions and why smaller individuals are evolutionarily favored under elevated temperatures.

The rise of pelagic sharks and adaptive evolution of pectoral fin morphology during the Cretaceous

The emergence and subsequent evolution of pectoral fins is a key point in vertebrate evolution, as pectoral fins are dominant control surfaces for locomotion in extant fishes.1,2,3 However, major gaps remain in our understanding of the diversity and evolution of pectoral fins among cartilaginous fishes (Chondrichthyes), a group with an evolutionary history spanning over 400 million years with current selachians (modern sharks) appearing about 200 million years ago.4,5,6 Modern sharks are a charismatic group of vertebrates often thought to be predators roaming the open ocean and coastal areas, but most extant species occupy the seafloor.4 Here we use an integrative approach to understand what facilitated the expansion to the pelagic realm and what morphological changes accompanied this shift. On the basis of comparative analyses in the framework of a time-calibrated molecular phylogeny,7 we show that modern sharks expanded to the pelagic realm no later than the Early Cretaceous (Barremian). The pattern of pectoral fin aspect ratios across selachians is congruent with adaptive evolution, and we identify an increase of the subclade disparity of aspect ratio at a time when sea surface temperatures were at their highest.8 The expansion to open ocean habitats likely involved extended bouts of sustained fast swimming, which led to the selection for efficient movement via higher aspect ratio pectoral fins. Swimming performance was likely enhanced in pelagic sharks during this time due to the elevated temperatures in the sea, highlighting that shark evolution has been greatly impacted by climate change.

Ontogenetic Plasticity in Shoaling Behavior in a Forage Fish under Warming

Shoaling behavior is known to increase survival rates during attacks from predators, minimize foraging time, favor mating, and potentially increase locomotor efficiency. The onset of shoaling typically occurs during the larval phase, but it is unclear how it may improve across ontogenetic stages in forage fishes. Warming is known to increase metabolic rates during locomotion in solitary fish, and shoaling species may adjust their collective behavior to offset the elevated costs of swimming at higher temperatures. In this study, we quantified the effects of warming on shoaling performance across the ontogeny of a small forage fish, zebrafish (Danio rerio) at different speeds. Shoals of larval, juvenile, and adult zebrafish were acclimated at two temperatures (28°C and 32°C), and metabolic rates were quantified prior to and following nonexhaustive exercise at high speed. Shoals of five individuals were filmed in a flow tank to analyze the kinematics of collective movement. We found that zebrafish improve shoaling swimming performance from larvae to juveniles to adults. In particular, shoals become more cohesive, and both tail beat frequency (TBF) and head-to-tail amplitude decrease with ontogeny. Early life stages have higher thermal sensitivity in metabolic rates and TBF especially at high speeds, when compared to adults. Our study shows that shoaling behavior and thermal sensitivity improve as zebrafish shift from larval to juvenile to adult stages.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Widespread alterations to hypothalamic-pituitary-gonadal (HPG) axis signaling underlie high temperature reproductive inhibition in the eurythermal sheepshead minnow (Cyprinodon variegatus)

Fish experiencing abnormally high or prolonged elevations in temperature can exhibit impaired reproduction, even for species adapted to warm water environments. Such high temperature inhibition of reproduction has been linked to diminished gonadal steroidogenesis, but the mechanisms whereby hypothalamic-pituitary-gonadal (HPG) axis signaling is impacted by high temperature are not fully understood. Here, we characterized differences in HPG status in adult sheepshead minnow (Cyprinodon variegatus), a eurythermal salt marsh and estuarine species of eastern North America, exposed for 14 d to temperatures of 27 °C or 37 °C. Males and females at 37 °C had lower gonadosomatic index (GSI) values compared to fish at 27 °C, and females at 37 °C had fewer spawning capable eggs and lower circulating 17β-estradiol (E₂). Gene transcripts encoding gonadotropin-inhibitory hormone (gnih) and gonadotropin-releasing hormone-3 (gnrh3) were higher in relative abundance in the hypothalamus of both sexes at 37 °C. While pituitary mRNAs for the β-subunits of follicle-stimulating hormone (fshβ) and luteinizing hormone (lhβ) were lowered only in males at 37 °C, Fsh and Lh receptor mRNA levels in the gonads were at lower relative levels in both the ovary and testis of fish at 37 °C. Females at 37 °C also showed reduced ovarian mRNA levels for steroid acute regulatory protein (star), P450 side-chain cleavage enzyme (cyp11a1), 3β-hydroxysteroid dehydrogenase (3βhsd), 17β-hydroxysteroid dehydrogenase (hsd17β3), and ovarian aromatase (cyp19a1a). Females at the higher 37 °C temperature also had a lower liver expression of mRNAs encoding estrogen receptor α (esr1) and several vitellogenin and choriogenin genes, but elevated mRNA levels for hepatic sex hormone-binding globulin (shbg). Our results substantiate prior findings that exposure of fish to high temperature can inhibit gonadal steroidogenesis and oogenesis, and point to declines in reproductive performance emerging from alterations at several levels of HPG axis signaling including increased hypothalamic Gnih expression, depressed gonadal steroidogenesis, and reduced egg yolk and egg envelope protein production in the liver.

Physiological mechanisms linking cold acclimation and the poleward distribution limit of a range-extending marine fish

Extensions of species' geographical distributions, or range extensions, are among the primary ecological responses to climate change in the oceans. Considerable variation across the rates at which species' ranges change with temperature hinders our ability to forecast range extensions based on climate data alone. To better manage the consequences of ongoing and future range extensions for global marine biodiversity, more information is needed on the biological mechanisms that link temperatures to range limits. This is especially important at understudied, low relative temperatures relevant to poleward range extensions, which appear to outpace warm range edge contractions four times over. Here, we capitalized on the ongoing range extension of a teleost predator, the Australasian snapper Chrysophrys auratus, to examine multiple measures of ecologically relevant physiological performance at the population's poleward range extension front. Swim tunnel respirometry was used to determine how mid-range and poleward range edge winter acclimation temperatures affect metabolic rate, aerobic scope, swimming performance and efficiency and recovery from exercise. Relative to 'optimal' mid-range temperature acclimation, subsequent range edge minimum temperature acclimation resulted in absolute aerobic scope decreasing while factorial aerobic scope increased; efficiency of swimming increased while maximum sustainable swimming speed decreased; and recovery from exercise required a longer duration despite lower oxygen payback. Cold-acclimated swimming faster than 0.9 body lengths sec-1 required a greater proportion of aerobic scope despite decreased cost of transport. Reduced aerobic scope did not account for declines in recovery and lower maximum sustainable swimming speed. These results suggest that while performances decline at range edge minimum temperatures, cold-acclimated snapper are optimized for energy savings and range edge limitation may arise from suboptimal temperature exposure throughout the year rather than acute minimum temperature exposure. We propose incorporating performance data with in situ behaviour and environmental data in bioenergetic models to better understand how thermal tolerance determines range limits.

Metabolic costs of spontaneous swimming in Sprattus sprattus L., at different water temperatures

Oxygen uptake (MO₂; mgO₂ fish^-1h^-1) of fish groups was measured at temperatures between 10–19°C in an intermittent-flow respirometer to quantify the metabolic costs of spontaneous swimming patterns in the small clupeid Sprattus sprattus. Movements of individual fish within the school were tracked automatically during respirometry. Oxygen uptake was then related to mean swimming speeds and the number of sharp turns (>90°), which are common behavioural elements of spontaneous swimming in clupeid fish. Different possible model formulations for describing the relationship between respiration and swimming patterns were compared via the AIC. The final model revealed that costs for sharp turns at a frequency of 1 s^-1 doubled the metabolic costs compared to those with zero turns but with likewise a moderate swimming speed of 0.28 body length ^-1. The cost for swimming doubled if the swimming speed was doubled from 0.28 to 0.56 BLs^-1 but increased by a factor of 4.5 if tripled to 0.84 BLs^-1. Costs for transport were minimal at a speed of 0.4 body lengths s^-1 at all temperatures. New basic input parameters to estimate energy losses during spontaneous movements, which occur typically during foraging in this small pelagic fish, are provided.

Excess postexercise oxygen consumption decreases with swimming duration in a labriform fish: Integrating aerobic and anaerobic metabolism across time

Many vertebrate animals employ anaerobic pathways during high-speed exercise, even if it imposes an energetic cost during postexercise recovery, expressed as excess postexercise oxygen consumption (EPOC). In ectotherms such a fish, the initial anaerobic contribution to exercise is often substantial. Even so, fish may recover from anaerobic pathways as swimming exercise ensues and aerobic metabolism stabilizes, thus total energetic costs of exercise could depend on swimming duration and subsequent physiological recovery. To test this hypothesis, we examined EPOC in striped surfperch (Embiotoca lateralis) that swam at high speeds (3.25 L s^-1 ) during randomly ordered 2-, 5-, 10-, and 20-min exercise periods. We found that EPOC was highest after the 2-min period (20.9 mg O₂  kg^-1 ) and lowest after the 20-min period (13.6 mg O₂  kg^-1 ), indicating that recovery from anaerobic pathways improved with exercise duration. Remarkably, EPOC for the 2-min period accounted for 72% of the total O₂ consumption, whereas EPOC for the 20-min period only accounted for 14%. Thus, the data revealed a striking decline in the total cost of transport from 0.772 to 0.226 mg O₂ ·kg^-1 ·m^-1 during 2- and 20-min periods, respectively. Our study is the first to combine anaerobic and aerobic swimming costs to demonstrate an effect of swimming duration on EPOC in fish. Clarifying the dynamic nature of exercise-related costs is relevant to extrapolating laboratory findings to animals in the wild.

A large predatory reef fish species moderates feeding and activity patterns in response to seasonal and latitudinal temperature variation

Climate-driven increases in ocean temperatures are expected to affect the metabolic requirements of marine species substantially. To mitigate the impacts of increasing temperatures in the short-term, it may be necessary for ectothermic organisms to alter their foraging behaviour and activity. Herein, we investigate seasonal variation in foraging behaviour and activity of latitudinally distinct populations of a large coral reef predator, the common coral trout, Plectropomus leopardus, from the Great Barrier Reef, Australia. P. leopardus exhibited increased foraging frequency in summer versus winter time, irrespective of latitude, however, foraging frequency substantially declined at water temperatures >30 °C. Foraging frequency also decreased with body size but there was no interaction with temperature. Activity patterns were directly correlated with water temperature; during summer, the low-latitude population of P. leopardus spent up to 62% of their time inactive, compared with 43% for the high-latitude population. The impact of water temperature on activity patterns was greatest for larger individuals. These results show that P. leopardus moderate their foraging behaviour and activity according to changes in ambient temperatures. It seems likely that increasing ocean temperatures may impose significant constraints on the capacity of large-bodied fishes to obtain sufficient prey resources while simultaneously conserving energy.

Cost of Tolerance: Physiological Consequences of Evolved Resistance to Inhabit a Polluted Environment in Teleost Fish Fundulus heteroclitus

Anthropogenic stressors, including pollutants, are key evolutionary drivers. It is hypothesized that rapid evolution to anthropogenic changes may alter fundamental physiological processes (e.g., energy metabolism), compromising an organism's capacity to respond to additional stressors. The Elizabeth River (ER) Superfund site represents a "natural-experiment" to explore this hypothesis in several subpopulations of Atlantic killifish that have evolved a gradation of resistance to a ubiquitous pollutant-polycyclic aromatic hydrocarbons (PAH). We examined bioenergetic shifts and associated consequences in PAH-resistant killifish by integrating genomic, physiological, and modeling approaches. Population genomics data revealed that genomic regions encoding bioenergetic processes are under selection in PAH-adapted fish from the most contaminated ER site and ex vivo studies confirmed altered mitochondrial function in these fish. Further analyses extending to differentially PAH-resistant subpopulations showed organismal level bioenergetic shifts in ER fish that are associated with increased cost of living, decreased performance, and altered metabolic response to temperature stress-an indication of reduced thermal plasticity. A movement model predicted a higher energetic cost for PAH-resistant subpopulations when seeking an optimum habitat. Collectively, we demonstrate that pollution adaption and inhabiting contaminated environments may result in physiological shifts leading to compromised organismal capacity to respond to additional stressors.

Global patterns in lake ecosystem responses to warming based on the temperature dependence of metabolism

Climate warming is expected to have large effects on ecosystems in part due to the temperature dependence of metabolism. The responses of metabolic rates to climate warming may be greatest in the tropics and at low elevations because mean temperatures are warmer there and metabolic rates respond exponentially to temperature (with exponents >1). However, if warming rates are sufficiently fast in higher latitude/elevation lakes, metabolic rate responses to warming may still be greater there even though metabolic rates respond exponentially to temperature. Thus, a wide range of global patterns in the magnitude of metabolic rate responses to warming could emerge depending on global patterns of temperature and warming rates. Here we use the Boltzmann-Arrhenius equation, published estimates of activation energy, and time series of temperature from 271 lakes to estimate long-term (1970-2010) changes in 64 metabolic processes in lakes. The estimated responses of metabolic processes to warming were usually greatest in tropical/low-elevation lakes even though surface temperatures in higher latitude/elevation lakes are warming faster. However, when the thermal sensitivity of a metabolic process is especially weak, higher latitude/elevation lakes had larger responses to warming in parallel with warming rates. Our results show that the sensitivity of a given response to temperature (as described by its activation energy) provides a simple heuristic for predicting whether tropical/low-elevation lakes will have larger or smaller metabolic responses to warming than higher latitude/elevation lakes. Overall, we conclude that the direct metabolic consequences of lake warming are likely to be felt most strongly at low latitudes and low elevations where metabolism-linked ecosystem services may be most affected.

Using insights from animal behaviour and behavioural ecology to inform marine conservation initiatives

The impacts of human activities on the natural world are becoming increasingly apparent, with rapid development and exploitation occurring at the expense of habitat quality and biodiversity. Declines are especially concerning in the oceans, which hold intrinsic value due to their biological uniqueness as well as their substantial sociological and economic importance. Here, we review the literature and investigate whether incorporation of knowledge from the fields of animal behaviour and behavioural ecology may improve the effectiveness of conservation initiatives in marine systems. In particular, we consider (1) how knowledge of larval behaviour and ecology may be used to inform the design of marine protected areas, (2) how protecting species that hold specific ecological niches may be of particular importance for maximizing the preservation of biodiversity, (3) how current harvesting techniques may be inadvertently skewing the behavioural phenotypes of stock populations and whether changes to current practices may lessen this skew and reinforce population persistence, and (4) how understanding the behavioural and physiological responses of species to a changing environment may provide essential insights into areas of particular vulnerability for prioritized conservation attention. The complex nature of conservation programmes inherently results in interdisciplinary responses, and the incorporation of knowledge from the fields of animal behaviour and behavioural ecology may increase our ability to stem the loss of biodiversity in marine environments.

Heterogeneity in prey distribution allows for higher food intake in planktivorous fish, particularly when hot

When prey are scarce, planktivorous fish and other predators feeding on tiny prey should forage within prey-rich patches to attain a net food intake above the ambient mean food concentrations. If they can indeed locate prey-rich patches efficiently, then a patchy distribution of planktonic prey should lead to: (1) an increase in the overall per capita food intake, and (2) greater variability among predators in prey-capture rate due to differences in arrival times. Both phenomena were observed in 34 daily feeding sessions with a cohort of juvenile rudd held in twin experimental systems, each housing the same number of fish free to move in a loop of ten interconnected 200-L tanks. The fish were fed daily with equal numbers of planktonic prey (Artemia nauplii), offered either in a homogeneous or patchy distribution. To simulate low and high temperatures that represent potential global warming scenarios, the feeding protocol was replicated at 16, 21 and 26 °C, on each occasion following a 3-day period of fish acclimation. Up to 40–70 % of fish in the system with the patchy prey distribution assembled rapidly in the high-prey-density tank, the capture rate of first arrivals being up to 60 prey min⁻¹ at 26 °C, orders of magnitude greater than that of latecomers. The overall capture rates were higher in the system with patchy prey, regardless of the temperature. At the highest temperature (26 °C), the fish located the high-prey-density tank in less than half the time taken at the lowest temperature (16 °C, Q₁₀ > 2).

Sport science for salmon and other species: ecological consequences of metabolic power constraints

For metabolically demanding behaviours, power supply (ATP resynthesis per unit time) is an important constraint on performance. Yet ecology as a discipline lacks a framework to account for these power constraints. We developed such a framework (borrowing concepts from sports science) and applied it to the upriver migration of anadromous fish. Our models demonstrate how metabolic power constraints alters optimal migratory behaviour; in response to strong counter flows, fish minimise cost of transport by alternating between rapid, anaerobically fuelled swimming and holding to restore spent fuels. Models ignoring power constraints underestimated the effect of elevated water temperature on migration speed and costs (by up to 60%). These differences were primarily due to a temperature-mediated reduction in aerobic scope that impairs the ability of fish to rapidly migrate through warm waters. Our framework provides a mechanistic link between temperature-induced reductions in aerobic scope and their ecological consequences for individuals, populations and communities.

Increasing ocean temperatures reduce activity patterns of a large commercially important coral reef fish

Large-bodied fish are critical for sustaining coral reef fisheries, but little is known about the vulnerability of these fish to global warming. This study examined the effects of elevated temperatures on the movement and activity patterns of the common coral trout Plectropomus leopardus (Serranidae), which is an important fishery species in tropical Australia and throughout the Indo West-Pacific. Adult fish were collected from two locations on Australia's Great Barrier Reef (23°S and 14°S) and maintained at one of four temperatures (24, 27, 30, 33 °C). Following >4 weeks acclimation, the spontaneous swimming speeds and activity patterns of individuals were recorded over a period of 12 days. At 24-27 °C, spontaneous swimming speeds of common coral trout were 0.43-0.45 body lengths per second (bls(-1)), but dropped sharply to 0.29 bls(-1) at 30 °C and 0.25 bls(-1) at 33 °C. Concurrently, individuals spent 9.3-10.6% of their time resting motionless on the bottom at 24-27 °C, but this behaviour increased to 14.0% at 30 °C and 20.0% of the time at 33 °C (mean ± SE). The impact of temperature was greatest for smaller individuals (<45 cm TL), showing significant changes to swimming speeds across every temperature tested, while medium (45-55 cm TL) and large individuals (>55 cm TL) were first affected by 30 °C and 33 °C, respectively. Importantly, there was some indication that populations can adapt to elevated temperature if presented with adequate time, as the high-latitude population decreased significantly in swimming speeds at both 30 °C and 33 °C, while the low-latitude population only showed significant reductions at 33 °C. Given that movement and activity patterns of large mobile species are directly related to prey encounter rates, ability to capture prey and avoid predators, any reductions in activity patterns are likely to reduce overall foraging and energy intake, limit the energy available for growth and reproduction, and affect the fitness and survival of individuals and populations.

Ecosystem size structure response to 21st century climate projection: large fish abundance decreases in the central North Pacific and increases in the California Current

Output from an earth system model is paired with a size-based food web model to investigate the effects of climate change on the abundance of large fish over the 21st century. The earth system model, forced by the Intergovernmental Panel on Climate Change (IPCC) Special report on emission scenario A2, combines a coupled climate model with a biogeochemical model including major nutrients, three phytoplankton functional groups, and zooplankton grazing. The size-based food web model includes linkages between two size-structured pelagic communities: primary producers and consumers. Our investigation focuses on seven sites in the North Pacific, each highlighting a specific aspect of projected climate change, and includes top-down ecosystem depletion through fishing. We project declines in large fish abundance ranging from 0 to 75.8% in the central North Pacific and increases of up to 43.0% in the California Current (CC) region over the 21st century in response to change in phytoplankton size structure and direct physiological effects. We find that fish abundance is especially sensitive to projected changes in large phytoplankton density and our model projects changes in the abundance of large fish being of the same order of magnitude as changes in the abundance of large phytoplankton. Thus, studies that address only climate-induced impacts to primary production without including changes to phytoplankton size structure may not adequately project ecosystem responses.

Energetic extremes in aquatic locomotion by coral reef fishes

Underwater locomotion is challenging due to the high friction and resistance imposed on a body moving through water and energy lost in the wake during undulatory propulsion. While aquatic organisms have evolved streamlined shapes to overcome such resistance, underwater locomotion has long been considered a costly exercise. Recent evidence for a range of swimming vertebrates, however, has suggested that flapping paired appendages around a rigid body may be an extremely efficient means of aquatic locomotion. Using intermittent flow-through respirometry, we found exceptional energetic performance in the Bluelined wrasse Stethojulis bandanensis, which maintains tuna-like optimum cruising speeds (up to 1 metre s(-1)) while using 40% less energy than expected for their body size. Displaying an exceptional aerobic scope (22-fold above resting), streamlined rigid-body posture, and wing-like fins that generate lift-based thrust, S. bandanensis literally flies underwater to efficiently maintain high optimum swimming speeds. Extreme energetic performance may be key to the colonization of highly variable environments, such as the wave-swept habitats where S. bandanensis and other wing-finned species tend to occur. Challenging preconceived notions of how best to power aquatic locomotion, biomimicry of such lift-based fin movements could yield dramatic reductions in the power needed to propel underwater vehicles at high speed.